The present invention relates to a coding circuit for a facsimile apparatus and, more particularly, to a circuit for encoding facsimile data by sequentially utilizing an interrelationship between the position of a changing picture element of a scan line which is being encoded and the position of a changing picture element on the scan line which is immediately preceding the line being coded.
Two-dimensional coding methods used heretofore for a facsimile apparatus include the two-dimensional, sequential processing, relative address coding method which utilizes an interrelation between a picture element or pixel on a currently coding scan line and a pixel on the preceding scan line. An example of such a method is the modified READ (Relative Element Address Designate) coding method which is the standard coding method for group 3 facsimile apparatus for document transmission according to the CCITT Recommendation T.4 (new) (see, "Telegraph and Telematic Services Terminal Equipment, Recommendations of the S and T Series," Vol. VII-Fascicle VII.2, CCITT, (1981), pp. 225-235).
A prior art circuit for the two-dimensional, sequential processing, relative address coding system includes a shift register for shifting, pixel by pixel, a video signal on a scan line which is being encoded, ("coding line"). A shift register is also used for shifting, pixel by pixel, a video signal on the immediately preceding scan line, ("reference line"). Such a prior art circuit is found in Japanese Patent Application Disclosure Sho 56-73974, issued June 19, 1981. A problem is encountered with this prior art coding circuit because the whole video signal on the reference line is shifted by the shift register for the detection of changing pixels. A substantial period of time is required for the detection of changing pixels on a reference line. As a result, there is a limitation on the speed available for the processing.